1. Field of the Invention
The present invention relates to a unit and a method for coating a substrate with a coating solution.
2. Description of the Related Art
In a photolithography process, for example, in semiconductor device fabrication processes, a resist coating treatment of coating the surface of a wafer with a resist solution to form a resist film, an exposure treatment of exposing a wafer in a pattern, a developing treatment of developing the exposed wafer, and so on are carried out in sequence to form a predetermined circuit pattern on the wafer.
The above-mentioned resist coating treatment is usually carried out by a resist coating unit. This resist coating unit has a spin chuck for horizontally holding the wafer by suction and rotating the wafer and a discharge nozzle which moves to a position above the center of the wafer to discharge the resist solution onto the wafer. At the time of the resist coating, the resist solution is discharged from the discharge nozzle onto the center of the rotated wafer and is diffused by a centrifugal force so that a resist film is formed on the surface of the wafer.
In order to diffuse the resist solution, the viscosity of the resist solution needs to be low and for this purpose, a predetermined amount of a solvent is included in the resist solution in advance.
When the resist solution is diffused while the wafer is rotated as described above, however, a circumferential speed in a region closer to an outer edge portion of the wafer becomes higher so that the solvent in the resist solution is volatilized before the resist solution reaches the outer edge portion of the wafer and the resist solution is not sometimes diffused appropriately. Even when it is diffused, an uneven resist film may possibly be formed over the wafer since a film thickness on the wafer center portion differs from that on the outer edge portion and a peripheral portion of the wafer.
When the uneven resist form is thus formed, portions usable as a semiconductor device are lessened to cause decrease in yield. Especially, since the amount of the resist solution supplied onto the wafer tends to be reduced in response to the recent demand for a thinner resist film and reduction in the amount of the resist solution, it is very important to maintain the viscosity of the resist solution supplied onto the wafer to form an appropriate film.